Molded parts in the context of the invention preferably include transparent molded parts, such as transparent sheets, lenses, optical storage media and carriers for optical storage media respectively, or also articles from the automotive glazings sector, such as for example headlight diffusers. The invention provides in particular optical storage media and carriers for optical storage media, such as for example writeable optical data storage media that exhibit a good coatability and wettability and that are suitable for example for the application of colorants from solution, in particular from non-polar media. In addition the optical injection-molded parts made from the polycarbonates according to the invention have a lesser tendency to become contaminated.
Transparent injection-molded parts are important in particular in the glazings and storage media sectors.
Optical data recording materials are increasingly used as variable recording and/or archiving medium for large amounts of data. Examples of this type of optical data storage media are CDs, superaudio-CDs, CD-Rs, CD-RWs, DVDs, DVD-Rs, DVD+Rs, DVD-RWs, DVD+RWs and BDs.
Transparent thermoplastic materials such as for example polycarbonate, polymethyl methacrylate and chemical modifications thereof are typically used for optical storage media. Polycarbonate as substrate material is suitable in particular for write-once optical discs and read-many optical discs as well as for write-many optical discs, and also for the production of molded parts in the automotive glazings sector, such as for example headlamp diffusers. This thermoplastic material has an excellent mechanical stability, is only slightly susceptible to dimensional changes, and is characterised by a high transparency and impact resistance.
Polycarbonate produced by the phase interface (interfacial) process may be used for the production of optical data storages of formats described above such as Compact Discs (CDs) or Digital Versatile Discs (DVDs). These discs often have the property that they build up a high electrostatic field during their production in the injection-molding process. This high electrostatic field on the substrate during the production of the optical data storage media attracts for example dust from the surroundings or causes the injection-molded articles to stick together, for example disks to stick to one another, which reduces the quality of the finished injection-molded articles and complicates the injection-molding process.
It is furthermore known that the electrostatic charge (in the form of electrostatic fields) in particular of discs (for optical data carriers) leads to an inadequate wettability in particular with non-polar media, such as for example a non-polar colorant or application of a colorant from solvents, such as for example dibutyl ether, ethylcyclohexane, tetrafluoropropanol, cyclohexane, methylcyclohexane or octafluoropropanol. Thus, a high electrostatic field on the surface of the substrate during the application of a colorant in the case of writeable data storages causes for example an irregular coating with the colorant and thus leads to defects in the information layer.
The degree of the electrostatic charging of a substrate material may be quantified for example by measuring the electrostatic field at a certain distance from the substrate surface.
In the case of an optical data storage in which a writeable layer is applied to the surface by a spin-coating process, a low electrostatic field is necessary in order to guarantee the uniform application of the writeable layer and ensure a problem-free production process.
In addition a high electrostatic field causes yield losses with regard to the substrate material on account of the above mentioned factors. This may lead to a stoppage of the respective production step and is associated with high costs. Many approaches have been followed in order to solve this problem of a high electrostatic charge. In general antistatics are added as additives to the substrate material. Antistatic polycarbonate compositions are described for example in JP 62 207 358-A. Here, inter alia, phosphoric acid derivatives are added as antistatics to the polycarbonate. EP 0922 728-A describes various antistatics such as polyalkylene glycol derivatives, ethoxylated sorbitan monolaurate, polysiloxane derivatives, phosphine oxides, as well as distearylhydroxyamine, which are used individually or as mixtures. Japanese application JP 62 207 358 describes esters of phosphorous acid as additives. Sulfonic acid derivatives are described in U.S. Pat. No. 5,668,202. In WO 00/50 488 3,5-di-tert.-butylphenol is used as chain terminator in the phase interface process. This chain terminator leads to a low static charge of the corresponding substrate material compared to conventional chain terminators. JP 62 207 358-A describes polyethylene and polypropylene derivatives as additives for polycarbonate.
The additives described above may however have a disadvantageous effect on the properties of the substrate material, since they tend to leach out from the material. Although this is a desirable effect as regards the antistatic properties, it may lead to the formation of deposites in the mold and/or inaccurate molding. Furthermore, the content of oligomers in the polycarbonate may also lead to poorer mechanical properties and to a reduction of the glass transition temperature. In addition these additives may cause secondary reactions. The subsequent “endcapping” of polycarbonate that has been obtained by the transesterification process is complicated and the results achieved are not optimal. Moreover, high costs are involved in introducing new terminal groups into the material.
The object therefore exists of providing a composition and a substrate material that satisfies the requirements of as low a electrostatic field as possible on the substrate surface and that avoids the disadvantages described above.
This object has surprisingly been achieved by restricting the content of specially structured carbamate compounds in materials that are used for the production of optical data storages. A certain amount of carbamate compounds in the substrate material may arise due to the addition of additives, impurities in precursors, or due to the production process itself.